Antiretroviral therapy (ART) provided to HIV-positive individuals can reduce transmission risk by 96%. In this context, identifying new strategies to optimize treatment as prevention (TasP) is a top Fiscal Year 2014 Trans- NIH Research Priority. There are important gaps in our knowledge of how to implement TasP in key populations including female sex workers (FSW). Effective TasP will require higher levels of ART adherence than those achieved in typical treatment programs. Mobile Health (mHealth) approaches employing interactive short message service (SMS) messages have shown promise as a method for improving ART adherence, leading to suppression of plasma HIV RNA. However, the ability of SMS interventions to support ART adherence in key populations like FSWs and unique clinical scenarios such as TasP has yet to be evaluated. To answer this important HIV treatment and prevention question, we will test the overarching hypothesis that in Kenyan FSWs, a theory-based, individualized, interactive SMS intervention emphasizing motivation to adhere to treatment will significantly reduce the prevalence of detectable plasma HIV RNA 6 months following ART initiation compared to standard care. Aim 1 will employ qualitative research and community participation to craft theory-based, individualized, culturally appropriate SMS messages grounded in constructs of the information-motivation-behavioral skills model. Aim 2 will compare the efficacy of the SMS intervention versus standard care for reducing the proportion of individuals with detectable plasma HIV RNA 6 months after ART initiation in a randomized, controlled trial (RCT) including 210 FSWs. Aim 3 will explore the mechanism of the intervention's effect by comparing perceptions of support and motivation to adhere to ART in intervention versus control subjects using an adaptation of the LifeWindows Information - Motivation - Behavioral Skills ART Adherence Questionnaire. An important product of this work will be a procedural manual to facilitate future research and implementation. We will also conduct a preliminary cost effectiveness analysis to enable comparison of costs and benefits in relation to other treatment and prevention interventions. Our multidisciplinary Kenyan and US team has experience conducting full-scale mHealth trials within the time frame and budget afforded by an R21. This work will provide collaborative research and training opportunities with Kenyan partners to build mHealth capacity in Mombasa County. This geographic location is defined by Kenya's National AIDS and STD Control Programme as a high-incidence cluster, making it a target area for intensive HIV prevention interventions. Successful completion of the RCT could demonstrate an efficacious strategy to support treatment and secondary HIV prevention in FSWs. Depending on the initial findings, these data will be used to inform development of funding proposals for a larger (multi-site, larger N) and longer (1-2 year follow-up) RCT, or to develop an implementation research proposal to bring the intervention to scale while further evaluating its effectiveness in a real-world implementation context.